5 Feb
2011
5 Feb
'11
11:45 a.m.
On 2011 Feb 5, at 2:05 AM, Pete Turnbull wrote:
On 05/02/2011 00:42, Brent Hilpert wrote:
I have heard that argument too, but I'm not at all convinced by it. It
strikes me more as 230V system users trying to justify why it's
'better', or 'not less safe'.
As someone with direct experience working with both voltages (relative
to ground), I'm not fond of being shocked by either level, but I can
tell you quite quickly which level I prefer to work around and 'prefer'
to come in contact with. It varies between individuals and conditions,
but 120V shocks for many are quite tolerable in the sense of it being
enough to warn you without producing damage (or as William was pointing
out, the difference between 'shock' and 'surprise'). IME, I can't say
the same thing about 240.
240 is double 120, by Ohm's law and impedance, lethal currents are more
'likely' or easily produced by the higher voltage.
Yes, one always has to be careful around both levels for both shock
concern and because there is a fair mount of energy behind both
(slipped tools producing big sparks and damage).
I'm not well acquainted with the UK/Euro rules history, but I'll put
forth that the UK/Euro 110/55 building-site requirements are evidence
of the point. It's nice that it actually lowers the V-to-ground to such
a low level, but I expect it exists because the 240V-to-ground is
considered unsafe, while in NA we get by safely enough with the
120V-to-ground on building sites.
The real-world difference between North American
240V and Euro 240V
is the maximum potential relative to ground:
NA: 120 VRMS, 170V peak
Euro: 240 VRMS, 340V peak
Of course, the lower potential to ground is precisely why NA uses the
split-phase 120/240: it is much safer for humans but you still have
240 for heavy loads.
I believe that's debatable.
Now, I expect most of us would agree that voltage below about 70-90V
peak (around 55V RMS) is much safer, on the grounds that while it
might give a person an unpleasant surprise, a shock, even across the
body, is unlikely to be fatal. Hence the SELV rules and our use of
RLV 110V AC centre tapped to ground on building sites, giving a
maximum of 55V RMS. (According to the standards bodies like IET, that
means a maximum around 40V for indirect contact shock protection, well
below the accepted standard safe touch voltage and therefore we don't
need to limit earth loop impedances and worry about RCDs on building
sites.)
Nevertheless, I've often heard it argued that voltages around 230V RMS
are safer than, say, 110V RMS, on the grounds that a casual brush with
a 230V live conductor will cause a very rapid reaction in humans,
often jerking the contact free, whereas voltages around the 100-150V
range don't. I can't remember where I read supporting evidence, but
it's certainly something I've often heard mentioned, and is one reason
we think American voltages are actually more dangerous than ours. All
that said, either can give a potentially fatal shock, as both are
sufficient to overcome initial skin resistance.